1. Field of the Invention
The invention relates to a wheel rim for a wheeled apparatus. More particularly, the invention relates to a wheel rim for a bicycle. The invention also relates to a wheel for a wheeled apparatus including the aforementioned rim, as well as to a method of manufacturing the rim.
2. Description of Background and Relevant Information
As is known, a rim for a bicycle wheel is formed as an annular profile having, inwardly, means for fastening compression spokes or tension spokes and, outwardly, an open channel for receiving a tire and, if necessary, a tube.
In a conventional spoke wheel, the rim represents about 60% of the weight of the wheel (for a front wheel) and 90% of the inertia.
Understandably, such circumstances have led manufacturers to the design of rims that are lightweight and rigid.
It should be noted, however, that the rim is a part of the bicycle that is subject to substantial forces. Indeed, with the exception of the tire; the rim is the component that is the closest to the ground and which, after the tire, is exposed to the ground unevenness and transmits the same to the remainder of the bicycle. If the tire is inadequately inflated, the rim can come in direct contact with the ground.
Moreover, the tire subjects the rim to high stresses due to its inflation pressure.
During a ride, each wheel rotation creates a load on each of the spokes, thereby causing mechanical fatigue cycles. The rim is reactionally subject to these fatigue cycles; therefore, it must be sufficiently strong to withstand these forces that can cause fatigue.
Finally, when braking, the rim must dissipate the heat generated by the friction of the pads, in the area of the lateral flanges of the rim.
The design of a rim must take into account all of the above-mentioned forces.
Generally speaking, the profile used for making a rim is characterized by its rigidity and strength.
Rigidity characterizes the more or less pronounced aptitude to elastic deformation. The rigidity of a material is evaluated by its modulus of elasticity (E). The higher the modulus, the more rigid the profile, and the less it deforms under a given load. The rigidity of a profile is evaluated by the product (E×I) of its modulus of elasticity E multiplied by the quadratic inertia (or moment) I of the profile.
Strength characterizes the solidity of the profile, that is, the maximum load which the profile can withstand prior to irreversible deformation, or the maximum load prior to breaking.
Furthermore, rigidity and strength are characterized along two preferred directions, i.e., first, the frontal direction for any force that tends to deform the rim in the plane that it defines, and second, the lateral direction for any force that tends to deform the rim transversely with respect to the plane that it defines.
In the field of bicycles, there are two large families of rims, rims for glued/cemented tubular tires and rims for pneumatic tires, the tires either requiring a tube or being tubeless.
Furthermore, two types of materials are normally used for the rims, namely, metal, such as steel in the past and an aluminum alloy currently, sometimes magnesium or titanium, on the one hand, and composite materials, more particularly carbon fibers, aramid fibers, or glass fibers embedded in a resin matrix, on the other hand.
The currently available aluminum alloy rims are economical, and they are well adapted to braking due to their friction and heat conduction properties.
In the field of aluminum alloy rims, developments have been undertaken with the goal of reducing the weight of the rims.
In this regard, the three documents EP714792 (and family member U.S. Pat. No. 5,651,591); EP715001; and EP1084868 (and family member U.S. Pat. No. 6,402,256) disclose lighter rims. In the first case, the particular shape of the rim casing makes it possible to reduce the thickness of the lower bridge. In the second case, the use of a higher performance alloy produces a rim with reduced weight following a chemical treatment that generally reduces the material thickness. In the third case, the lower bridge is machined between the zones for fastening the spokes so as to reduce the thickness of the bridge in these zones where the stresses have been observed to be relatively low.
In general, aluminum rims are made of alloys from the 6000 and the 7000 series. These alloys are differentiated by the types of materials and amounts of materials that are added. These loadings make it possible to improve the mechanical properties of the alloy and, therefore, to reduce the thickness of the walls of the rim case.
However, the modulus of elasticity is almost constant for all of the aluminum alloys; it is on the order of 69 500 MPa (Mega Pascals) for a density on the order of 2.7. This means that all of the aluminum alloys almost have the same modulus. The amount of loading only affects the strength of the alloy. Therefore, a lighter rim made out of a top of the line alloy is stronger while keeping the same rigidity as a rim made of a standard alloy.
Furthermore, the more heavily loaded the alloy, the more difficult it is to extrude. And the extrusion speed is directly linked to the wall thickness. Under these conditions, it is easily understood that the use of more heavily loaded aluminum alloys yields good results in terms of strength but not in terms of rigidity, and leads to technical solutions that are relatively complex to implement and ultimately uneconomical.
The composite materials have the advantage of having very good mechanical properties, as well as a very good anticorrosion property. By way of comparison, the modulus of a unidirectional carbon epoxy composite varies between 100,000 and 500,000 MPa (for a 1.6 density), as a function of the specific modulus of the fibers and of the resin content. However, their drawback lies in their cost, their mediocre braking properties, particularly in the rain, and heat conduction properties, their mediocre resistance to complex biases, as is the case in a tire rim, and a mediocre resistance to impacts due to the low shear strength of composites.
There are also rims that have a metallic portion and a portion made of a composite material. These rims are normally constructed with a metallic outer band having an annular channel for the pneumatic tire and housings for fastening the spokes, as well as a thin inner ring made of a composite material. The inner ring makes it possible to reduce the weight of the metallic portion while improving the aerodynamics of the rim.
However, these rims are relatively complex to manufacture. Moreover, the contribution of the composite portion to the mechanical properties of the rim is not optimized because bidirectional fiber layers are used.
Another known rim is described in U.S. Pat. No. 6,767,070. This rim is formed by four rings connected to one another by spacers. The spokes are fastened to the spacers. Although the rim has an original structure, it is not rigid because the rings have only localized connections therebetween. There is no actual cohesion of the overall structure.
In view of the above prior art, there is a need for a rim made from a shaped element, or profile, which is easy to manufacture and has improved mechanical properties in relation to the currently available traditional rims.